1. Field of the Invention
The present invention relates to a method for fabrication of an organic electroluminescent device utilizing an electroluminescent (EL) phenomenon produced in organic substances.
2. Related Art
Known electroluminescent (may hereinafter be referred to as EL) devices include organic and inorganic EL devices. In particular, the organic EL devices have come into widespread use in displays for their capability to be driven at low voltages.
FIG. 4 is a sectional view, illustrating a prior art display utilizing the organic EL device. Overlying a glass support 1 is a parallel set of strip-form transparent electrodes 2 which extends in a direction perpendicular to a paper surface of FIG. 4. The parallel set of transparent electrodes 2 constitutes a first electrode. An organic EL layer 7 is deposited over an entire surface of the support 1 inclusive of the transparent electrodes 2. Formed on top of the organic EL layer 7 is another parallel set of strip-form back electrodes 6 constituting a second electrode and oriented to intersect the set of transparent electrodes 2. One EL cell is defined between one transparent electrode 2 and one intersecting back electrode 6 to provide one pixel.
The organic EL layer 7 is constructed by sequentially stacking an organic hole injecting and transporting layer, a luminescent layer and an organic electron injecting and transporting layer.
Since the organic EL layer 7 has a reduced thickness relative to the transparent electrodes 2, the organic EL layer 7 is relatively thinned at its portions overlying side slopes of the transparent electrodes 2, as indicated by "N" portions in FIG. 4. Those "N" portions become thinner particularly when the organic EL layer 7 is deposited by a dry process, such as vacuum vapor deposition, wherein the organic EL layer 7 is deposited along irregular profiles of the transparent electrodes 2. On the other hand, the organic EL layer 7 is relatively thickened at its portions overlying leveled central surfaces of the transparent electrodes 2, as represented by an "M" portion, where it exhibits increased resistance relative to "N" portions. As a result, in each pixel region, a relatively higher current flows in the thinner "N" portions than in the thicker "M" portion. This causes the "N" portions of the organic EL layer 7 to emit brighter light, leading to inhomogeneous emission as well as the reduction in dielectric strength of the organic EL layer.
The back electrodes 6 are also irregularly surfaced, as shown in FIG. 4, to possibly result in the occurrence of damages at elevated portions thereof or of cracks therein.
One possible measure to overcome such problems would be to deposit the organic EL layer on an electrode-embedded flat surface. In Japanese Patent Laying-Open No. Sho 57-185607 (1982), a technique is disclosed which can fabricate a patterned electrode having a flat surface.
FIG. 5 is a sectional view, showing a series of processes for fabrication of the patterned electrode disclosed in the above-identified publication. As shown in FIG. 5(a), a layer of transparent electrode 2 is first formed on a glass support 1. Then, a patterned photoresist 9 is formed on the layer of transparent electrode 2, as shown in FIG. 5(b). By a wet process using an etching solution, the layer of transparent electrode 2 is etch removed from the regions where there is no photoresist 9, so that the pattern of the photoresist is transferred to the transparent electrode 2, as shown in FIG. 5(c).
As shown in FIG. 5(d), an insulation layer 8 is deposited to overlie the patterned transparent electrode 2 and the remaining open regions of the glass support 1. A thickness of the insulation layer 8 is selected to approximate to that of the transparent electrode 2.
The publication states that the patterned electrode having a flat or smooth surface, as shown in FIG. 5(e), can be formed by subsequent removal of the photoresist 9 together with portions of the insulation layer 8 overlying the photoresist 9, by plasma etching or by invasion of a stripping solution.
However, the plasma etching is not considered to be an actually effective technique to remove the photoresist 9 and the overlying insulation layer 8 portions, because of the following reasons.
That is, a total thickness of the photoresist 9 and insulation layer 8 overlying the patterned transparent electrode 2 actually becomes larger than a thickness of the insulation layer 8 overlying the glass support 1 regions where there exists no transparent electrode 2. Accordingly, the removal of the photoresist 9 and insulation layer 8 from regions overlying the patterned transparent electrode 2 by plasma etching results in the simultaneous removal of surface portions of the insulation layer 8 overlying the glass support 1 regions where there is no transparent electrode 2. The attempt to form such a patterned electrode having a flat surface as shown in FIG. 5(e) thus fails.
Accordingly, the use of a stripping solution would be a necessary selection for removal of the photoresist 9 and insulation layer 8 from regions overlying the transparent electrodes 2. Where the stripping solution is used, a slight amount of impurities often remains on the transparent electrodes 2. The subsequent formation of an organic charge injecting and transporting layer thereon creates a problem that those impurities interfere with charge injection from the transparent electrodes to the organic charge injecting and transporting layer to thereby degrade characteristics. Examples of such impurities include high-molecular weight ingredients contained in the photoresist, water contained in the stripping solution and the like. These impurities deposit on transparent electrode surfaces in the form of physical and/or chemical adsorption.
Accordingly, the method disclosed in the above-identified publication is required to include a process of removing a slight amount of impurities left on the electrode surfaces after the formation of flat-surfaced, patterned electrode with the use of stripping solution. This complicates a manufacturing process of organic EL devices, as well as imposing a problem of increased cost.